In metal-oxide-semiconductor (MOS) monolithic integrated circuit technology, field-effect transistors (FET's) are the essential components, acting as both active and passive components. MOSFET's are formed with p-type channels or n-type channels. It is desirable at times to operate these devices in a depletion mode. Typically, depletion occurs when the channel is heavily doped with an appropriate species to form, simplistically speaking, an electrical short in the channel between the source and drain regions of the FET. This results in the device being "on", even though the applied voltage to the gate is zero.
This depletion mode affects the threshold voltage of the transistor. The threshold voltage is that voltage which must be applied to result in mobility of the electronic charge carriers across the channel. The threshold voltage is dependent on many characteristics of the FET including not only the doping density of the channel region or depletion zone, but also the work function difference between the gate and the silicon substrate, the oxide thickness and permittivity, the surface-state charge density and the distributed charge in the oxide.
Generally, variations in device threshold voltages, such as that caused by depletion, are preferably achieved through separate dopant implants into the channel of the device. This requires additional process steps and masks. In particular, the depletion zone is typically formed using ion implantation techniques wherein an appropriate species is implanted into the channel region to form the depletion zone. However, a shortcoming associated with this method as stated previously, is that generally additional mask and implant steps are required to form this depletion zone. As widely known throughout the electronics industry, it is desirable to minimize or alleviate the number of processing steps, particularly those that require masking and alignment procedures.
Therefore, it would be desirable to provide a method for forming a depletion device within an MOSFET which does not require the additional processing steps associated with the conventional masking and implanting methods for forming these devices. It would be extremely desirable if such method were compatible with currently utilized processing steps.